The two teams have had their fair share of problems in attack this season and must address fresh obstacles ahead of Sunday's Primera Division fixtures

ANALYSISBy Ben Hayward | Spanish Football Writer

It can be tough at the top. Problems come in all shapes and sizes for sides like Barcelona and Real Madrid; troubled recently in defence, Spain's biggest two teams are now facing new issues - this time in attack.

Barcelona suffered surprise defeat at Celtic on Wednesday night, when Lionel Messi scored a late consolation but saw his strike pale into insignificance in a 2-1 loss as the Catalans went home empty-handed for only the second time this season.

Messi's heated exchange with substitute David Villa late in the game showed the frustration felt on the night by Tito Vilanova's team, whose only previous defeat this term had come in the Spanish Supercopa second-leg against Real Madrid.

It was Messi and Villa's second spat in public this season, after the two argued on the pitch in the laboured home win against Granada back in September. However, press reports of a rift appear far-fetched. The real frustration, it seems, is that goals have been few and far between from the club's other forwards in 2012-13.

Messi has 18 in all competitions this season already and Villa, who is being eased back into action after he was seriously injured last December, has netted six so far in just 432 minutes. The side's other forwards, however, have contributed just five goals in a combined 2,544 minutes, which is putting extra pressure on Messi and - when he is on the pitch - Villa.

Vilanova has shown his faith in Pedro this term and although the Tenerife-born winger has looked much more consistent than in 2011-12, he has been unable to show it in front of goal, with just two strikes so far. Cristian Tello also has two, albeit in just 467 minutes, while Alexis Sanchez has netted just once in almost double the time, in what has been a hugely disappointing start to the campaign for the Chilean.

With Isaac Cuenca still sidelined, Vilanova has few alternatives, but can count on either Andres Iniesta or Cesc Fabregas further forward if needed.

In some games, the Catalans may be better off using Villa in the centre and reverting Messi to his former role on the right. In 2008-09, still the best Barca side in memory, the goals were spread out fairly evenly among Messi, Samuel Eto'o and Thierry Henry. Not now.

Messi scored an incredible 73 goals last season but, on the rare occasion he is unable to deliver, Barcelona have a problem - and that's something Vilanova must think about over the next few weeks, starting at Mallorca on Sunday night.

Real Madrid, meanwhile, are missing both Gonzalo Higuain and Karim Benzema for Sunday's tough trip to Levante, one of only two Liga fixtures Jose Mourinho's men lost in 2011-12.

Benzema vented his frustration at a lack of minutes in a recent interview with L'Equipe but was told to get on with the job in hand by Mourinho, who has shared the playing time fairly evenly between the Frenchman and Higuain this term.

With the Argentine out, Benzema has his opportunity to stake a claim for more minutes, although not on Sunday as he, too, misses the trip to the Valencia-based opposition through injury.

Mourinho also faces a dilemma because neither of his two strikers have completely convinced in 2012-13. The healthy competition from last term has been lost somewhat as the two men have blown hot and cold in the current campaign.

The Portuguese revealed on Saturday that Ronaldo will start as the main striker against Levante, with the 27-year-old set to drop deep in a similar role to Messi's at Barcelona - albeit within a very different system.

Ronaldo, like Messi, has 18 goals this season, although Higuain and Benzema have chipped in with eight and five respectively.

Being able to call on Cristiano is a wonderful option for any coach, but Mourinho will want more from his two strikers as the season wears on, especially as Benzema has said he wants to end his career at Madrid and Higuain is close to agreeing a new long-term deal with the capital club.

This weekend, with winnable matches for both Barca and Madrid, it is likely to be all about Messi and Ronaldo again. The trouble is, though, it cannot always be like that. Food for thought, then, for both Vilanova and Mourinho.